SO2 and NO2 are the critical precursors in forming sulfate and nitrate in ambient particles. We studied the mechanism of sulfate and nitrate formation during the co-uptake of NO2 and SO2 into NaCl droplets at different RHs under irradiation and dark conditions. A significant formation of nitrate attributable to NO2 hydrolysis was observed during the NO2 uptake under all conditions, and its formation rate increases with decreasing RH. The averaged NO2 uptake coefficient, γNO2, from the unary uptake of NO2 into NaCl droplets under dark conditions is 1.6 × 10−5, 1.9 × 10−5, and 3.0 × 10−5 at 80 %, 70 %, and 60 % RH, respectively. Chloride photolysis and nitrate photolysis play a crucial role in sulfate formation during the co-uptake. Nitrate photolysis generates reactive species (e.g., OH radicals, NO2, and N(III)) that directly react with S(IV) to produce sulfate. The OH radicals generated from nitrate photolysis can also react with chloride ions to form reactive chlorine species and then sulfate. To parameterize the role of nitrate photolysis and chloride photolysis in forming sulfate, the SO2 uptake coefficient, γSO2, as a function of the nitrate photolysis rate, PNO3- (jNO3- × [NO3-]), and chloride photolysis rate, PCl- (jCl- × [Cl−]), was derived as γSO2 = 0.41 × PNO3- + 0.34 × PCl-. Our findings open up new perspectives on the formation of secondary aerosol from the combined effect of nitrate photolysis and chlorine chemistry.